1. Field of the Invention
The present invention relates to a thermal printing sheet and in particular, to a thermal printing sheet having a superior running feature and a superior preservation stability.
2. Description of the Prior Art
The thermal method using sublimating dyes transfers a plenty of color dots by heating within a significantly short time so as to express a full color image using the color dots of various colors.
In this thermal method, as a thermal printing sheet is used a so-called sublimating heat-transfer sheet consisting of a substrate sheet made from a polyester film or the like, on one side of which are formed dye layers made from sublimating dyes and binder.
In the aforementioned thermal method, the thermal printing sheet is heated from its back by a thermal head according to the image information, so as to transfer the dye from the dye layers onto a printing paper, forming an image.
When carrying out a printing on a printing paper by using the thermal printing sheet, a heat is applied to the heat resistant slide layer from the thermal head so as to transfer dye from the dye layers of the thermal printing sheet onto the printing paper, the color formation concentration is in proportion to the heat quantity, according to which the surface temperature of the thermal head varies by several hundred degrees. For this, while the thermal printing sheet moves along the thermal head, the friction coefficient between the thermal head and the heat resistant slide layer is easily changed according to the temperature change. If the friction coefficient between the thermal head and the heat resistant slide layer changes, the thermal printing sheet cannot travel at a constant velocity, disabling to obtain a clear image.
For example, when the friction coefficient is large, the thermal printing sheet temporarily moves at a reduced speed, for which portion the concentration is increased, causing a so-called sticking (line-shaped printing disorder).
In order to prevent this sticking, it is necessary to reduce the friction coefficient especially at a higher temperature. Conventionally, phosphoric ester has been used as a lubricant for reducing the friction coefficient under a high temperature.
However, the phosphoric ester is a strong acid and the use of this phosphoric ester in the heat resistant slide layer causes various troubles are caused.
For example, when a thermal printing sheet is preserved in a rolled state, the dye layers are brought into contact with the heat resistant layer, and especially when preserved at a high temperature, the phosphoric ester dissolves a portion of the dye from the dye layers, causing a concentration decrease during a printing as well as a printing unevenness.
Moreover, when a lubricant of a strong acidity such as phosphoric ester is used, a dye which is easily subjected to a chemical reaction including decomposition in an acid environment such as indoaniline pigment is easily deteriorated, causing a color formation change and decreasing the transfer concentration.